1. Field of the Invention
The present invention relates to an image display apparatus which realizes stereovision.
2. Description of Prior Art
In a prior art three-dimensional image display apparatus, a display screen is divided into small sections, and each section is set for right or left eye according to predetermined rules. An image for right eye is displayed for the sections for right eye, and by using a slit-like light-shielding plate, a renticular lens or the like, the image comes only along the direction to right eye. Similarly to right eye, an image for left eye is displayed for the sections for left eye, and the image comes only along the direction to left eye. Stereovision is realized by viewing these images with the two eyes at the same time.
In this approach, the image viewed by the two eyes become a rough image through a lattice-like filter, and the image is liable to be lambent for the eyes. Further, because images for right eye and for left eye are displayed at the same time for each section in the display screen, it is necessary to mix the images into one image. The apparatus is operated as follows: The image for right eye is displayed for a predetermined time, and that for left eye is displayed for the same time thereafter, the image for right eye is displayed again, and so on. The operation for changing the image is repeated briefly to display images for right eye and for left eye intermittently. Then, the image viewed by the eyes becomes intermittent, and this enhances the lambency for the eyes.
Other approaches explained below are also proposed. For example, by putting on eyeglasses having polarizing filters, color filters or the like, a screen is observed, and different images are provided for the right and left eyes. By putting on a head-mount display, different screens are observed by right and left eyes. However, these need a special tool to be put on the eyes. Then, when it is used for a long time, the viewer's body is affected largely. For example, the viewer is tired. In the approaches using the eyeglasses, the images for the right and left eyes are projected on a screen at the same time, and it is necessary to mix the two images before displaying them on the screen. In the approach using the head mount display, the images of an object viewed commonly at the right and left sides are observed from different screens. Therefore, the eyes have a large burden for uniting the images. The eyes have another large burden for focus control because the distance between the eye and the screen is different largely from that between the eye and the object.
Further, an apparatus for stereovision using a hologram is proposed. In an apparatus for displaying a stereoscopic image described in Japanese Patent laid open Publication 6-342128/1994, images displayed by two image display devices are presented as a virtual image with a hologram lens. The hologram lens has two optical powers in correspondence to the right and left eyes of the viewer, and interference fringe is formed on the hologram lens in an area where the effective light beams for the right and left eyes overlap each other. In an example shown in FIG. 5 in the publication, an image display device has at least two elements for displaying an image for right eye and another image for left eye. In this approach, in the area where the effective light beams for the right and left eyes overlap each other, an image for left eye and that for right eye are provided for the left and right eyes from the same position on the hologram lens, and a three-dimensional image can be viewed in a situation similar to actual stereovision. Further, because different image display elements are provided for right eye and for left eye as the image display devices, it is not necessary to unite the two images before displaying them on the screen.
In an apparatus for stereovision described in Japanese Patent laid open Publication 4-355747/1992, a pair of cameras receive images of the same object, as shown in FIG. 1 of the publication. Then, a pair of cathode-ray tubes project the images received by the cameras, and a pair of lenses focus the projected images at a predetermined position. A hologram screen of transmission type or reflection type is provided at the focal position to diffract the pair of the images in predetermined directions and to condense the images to the positions of the two eyes. The hologram screen is used only as a grating.
In this approach, the left and right eyes can view different images with different visual points for the left and right eyes at the same time independently at the same predetermined positions on the hologram screen, and a three-dimensional stereoscopic image of an object can be viewed in a circumstance similar to natural stereovision. It is not necessary to unite the two images before displaying them on the screen similarly to the previous approach described in Japanese Patent laid open Publication 6-342128/1994, where different image display elements are provided for right eye and for left eye in the image display device.
However, the above-mentioned apparatuses for stereovision using a hologram have following problems. In the apparatus for stereovision described in Japanese Patent laid open Publication 4-355747/1992, a projected image of the display device is enlarged and focused on a hologram screen. Therefore, the focused image is distorted at the perimeter thereof if compared with the true image.
Further, in the apparatus for stereovision described in Japanese Patent laid open Publication 4-355747/1992, because the diffracted light is condensed, the positions of the eyes cannot be displaced from the positions where the light is condensed. Further, the apparatus cannot m6 be fitted for the difference in the distance between the right and left eyes of a viewer. The image display apparatus described in Japanese Patent laid open Publication 6-342128/1994 has similar problems because the diffracted light is condensed at a particular position.
Further, in the apparatus for stereovision described in Japanese Patent laid open Publication 4-355747/1992, and in the image display apparatus described in Japanese Patent laid open Publication 6-342128/1994, an imaginary image of the condensing point is reproduced at a point symmetrical to the condensing point relative to the plans of the hologram screen or the hologram lens, for a hologram where the diffracted light is condensed. Then, the light is adazzle there and the image is hard to be viewed.
Further, in the image display apparatus described in Japanese Patent laid open Publication 6-342128/1994, an image of the image display element for providing different images for right and left eyes is enlarged by the hologram lens as an imaginary image for the viewer's eyes located near the hologram lens. Therefore, for a large-scale image display apparatus, an entire stereoscopic image cannot be viewed by the eyes of a viewer distant from the hologram lens.
Further, in the apparatus for stereovision described in Japanese Patent laid open Publication 4-355747/1992, and in the image display apparatus described in Japanese Patent laid open Publication 6-342128/1994, a plurality of viewer cannot observe the same stereoscopic image. Further, a viewer cannot observe a three-dimensional image in a wide field of view. Further, a three-dimensional image of color display cannot be observed. Further, a three-dimensional image having a difference in visual points in the vertical direction cannot be observed. Further, when a viewer moves the positions of the eyes back and forth in the depth direction, a three-dimensional image having correct visual points cannot be observed.